Section 1 Mendelian Genetics Biology Unit 1 Review Genetics the study of the basis of biological variation and how biological characteristics are inherited Blending Theory of Inheritance the characteristics of each parent are blended in the offspring yielding an offspring intermediate between the 2 parents Problem predicted that offspring would ALWAYS be intermediate between their parents Doesn t explain how new traits appear Doctrine of the Fixity of Species each species of animal and plant was created individually by God and was fixed and unchanging in its characteristics Mendel s Particulate Theory of Inheritance inheritance is due to the passing of discrete particulate Genes are responsible for a single trait and are passed UNCHANGED from one generation to factors from parents to offspring the next Zygote fertilized egg Flowers and pea plants contain both male and female reproductive structures Pollination the male organ anther releases pollen The pollen is then carried by wind or by insects to the tip of the stigma female reproduction Cross pollination when pollen is carried from one plant to another by wind or by insect A hybrid is when plants of different varieties are crossed and displays a new mixture of traits Mendel s Peas the traits he studied were flower color plant height seed color and seed shape Pure bred red x pure bred white 100 red hybrids Red hybrids self pollinating 75 red 25 white Mendel s Law of Random Segregation 1 Each trait is controlled by a pair of genes alternative forms of the same trait alleles In each pair of alleles one is dominant and one is recessive In the formation of sex cells each allele of a gene pair segregates or separates from the other allele 2 3 in a random fashion Organisms that show a recessive phenotype always carry 2 recessive alleles The chance that 2 plants would both have white flowers is x 1 16 The chance that both plants would have red flowers would be x 9 16 Section 2 Chromosomes Cells Inheritance Mendel s work was published in 1865 but people were not ready to accept it because Most people believed in the Blending Theory of Inheritance Mendel s Particulate Theory of Inheritance implied that species could evolve through crossbreeding and the concept of biological evolution conflicted with the religious Doctrine of Biologists of the time did not understand that fertilization was due to the union of sperm and the Fixity of Species eggs Chromosomes paired structures in the nucleus of a cell Contain DNA the genetic material Walter Sutton showed that the paired chromosome structures behaved exactly as Mendel predicted for his particulate factors The pairs of chromosomes separated during the formation of eggs and sperm each egg and each sperm gets one chromosome from each chromosome pair Nucleus is the round central body Where the metabolism is The nucleus is surrounded by a gelatinous cytoplasm containing many other structures The entire cell is held together by an outer membrane called the cell membrane Human cells each contain 46 chromosomes 23 pairs of homologous chromosomes Gametes sex cells only contain a single copy of each chromosome haploid cells Somatic Cells cells that contain two copies of each chromosome diploid cells Divides to form 2 daughter cells Each daughter cell gets a complete and exact copy of ALL the genetic information in the parent Mitosis somatic cells the cells are diploid throughout and each daughter cell is genetically identical to the parent cell One cell produces 2 identical daughter cells daughter cells have a complete set of the cell original cell s genes Each cell is replicated then chromosomes condense and become visible in the nucleus Replicated chromosomes each consist of two arms called chromatids joined by a centromere Sister chromatids are made up of DNA and are exact duplicates Meiosis gametes one cell divides twice and produces 4 daughter cells and each has half as many chromosomes results as a haploid cell random segregation Division 1 homologous chromosomes are separated separates pairs and the cell itself divides in two creating two haploid daughter cells Division 2 sister chromatids are separated and each of the daughter cells divide again Produces 4 cells each of which is haploid and contains only one member of each homologous pair Chromosomes that are homologous are very similar to each other because they carry genes for the same traits but they are NOT necessarily exact copies of each other because they can carry different alleles Section 3 Expansion of Mendel s Principles Multiple Alleles ABO blood types Partial Dominance Codominance heterozygous individuals express both alleles and thus display an intermediate phenotype Pairs of codominant alleles lead to 3 classes of phenotypes not 2 Neither allele is dominant or recessive Example pure bred red x pure bred white pink flowers Then when the F1 pink carnations are crossed red pink and white flowers are bred in the F2 generation Polygenic Traits 2 or more gene pairs control a trait Continuous variation many traits controlled by several genes all acting together which results in a range of possible phenotypes Ex height in humans if you measured the entire population s height there would be no clear distinction between tall and short people Many traits that are polygenic are determined partly by our genes and partly by our environment Twin studies identical twins are called monozygotic because they are formed when the zygote splits in half just after the first cell division Fraternal twins are called dizygotic because when the woman produces two eggs during one menstrual cycle and both these eggs happen to be fertilized by two different sperms If both members of a pair of twins share a trait they are said to be concordant for that trait If they do not share the trait they are said to be discordant for the trait Sex determination all human cells except for gametes have 23 pairs of chromosomes Human females have XX human males XY sex chromosomes All other body chromosomes are called autosomes X chromosome carries many genes and is responsible for the production of lots of our physical traits embryo as a male Y chromosome has Testes Determining Factor in which results in the development of the Sex Linked Traits traits that are found more often in males than in females The genes for these traits are located on the X chromosome sex linked recessive Females have XX chromosomes so they have to be homozygous to display a recessive allele on
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